Wherever you go in the world, and as far back as 4,000 BC, stone is the common ingredient in roads. Simple stone roads were often rough, uneven, and pitted with ruts and holes that filled up with rain and mud in the winter. It wasn't until the 1700s that the smooth, even roads we know today became possible. We have three Scottish engineers and their improved road building techniques to thank. Although he was blind, John Metcalfe was able to design and build firm, three-layer roads. First he placed large stones on the bottom layer, then he took the materials excavated from the roadbed such as smaller rocks and earth and used them for the middle layer, and finally he spread a layer of gravel on top. A second Scottish gentleman by the name of Thomas Telford designed a way to raise the centre of the road so that rainwater would drain down the sides. He also devised a method to analyse how thick the road stones had to be to withstand the weight and volume of the horses and carriages that were common in his day. The last of the three, John McAdam, mixed the necessary road stones with tar. The tar "glued" all the stone together and created a harder and smoother surface for the carriage wheels to roll on. "Tarmacadam roads" became the standard used everywhere until the 1870s when asphalt took over.